MERRA-2 Notes, Questions, and Discussion

Submitted by mapr8844
on Wed, 08/17/2016 - 13:17

Notes:

For users: Most users will want to use assimilation fields and not analysis. The MERRA File Specification Document spells out the differences. The analysis states are not adjusted by the model, and are derived from the data assimilation system. These may be better for use as regional model lateral boundary conditions.

When you download data, there are streams (year ranges for different runs). For ftp access, you need to change the stream number in the files you download according to the following: 1980-1991: 100, 1992-2000: 200, 2001-2009: 300, 2010-present: 400.

When you click on the correct variable, it will take you to a second webpage with tabs that you can click that include: (1) documentation papers you need to cite, and (2) the correct variable citation information. (And you apparently need both types of citation). For example, here is a link to the page for 3d assimilated, 3-hourly, model level data: https://disc.gsfc.nasa.gov/datasets/M2I3NVASM_V5.12.4/summary?keywords=%22MERRA-2%22

Hi, I downloaded MERRA 6-hourly data of Specific humidity, Temperature and Wind and I cannot read them. I've tried to read them from Panoply and Matlab and I couldn't have access to the data. Two years ago I downloaded similar data and I could read them, but now I cannot, Should have it suffered any change the format? or any idea what is happening? Thank you.

Re: Access to data

Submitted by michael.bosilovich on Fri, 05/27/2016 - 07:43.

Hi Johanna,

A couple things here. It's very hard to answer your question, since thre is not much information about how you downloaded the data and what exactly the problem may be. First thing to do is to check the file sizes are appropriate. very small files may indicate a corrupted download. Also, this page is not the most efficient for getting a response from the data center or the GMAO. This type of question should go to the GES DISC help page, as it pertains to accessibility of the data.

In particular, the File Specification Document, will tell you everything you need to know about our native data formats and the grid structures (these may be manipulated by GES-DISC utilities though).

Lastly, since you are reading data with third party software, I will mention that there is no guarantee that our help emails will be able to provide assistance with those software. Best is to refer to their documentation and help lines.

Hi- The most likely the reason you can't read the data is that it is netCDF4 and not netCDF3. the files now end in nc4 and if you do a ncdump on a file from the ftp submitter NASA has you get, for example, ncdump -k svc_MERRA2_100.instM_2d_asm_Nx.198001.nc4 netCDF-4 classic model Both Panoply and MATLAB can read netCDF4 but you may need to upgrade your netCDF library. I checked some MERRA files that I have and I believe there were netCDF3. Cathy

Re: Access to data

Submitted by gilbert.p.compo on Thu, 05/26/2016 - 11:01.

Dr. Yepes, Would you send the precise path you used for the download, as well as the name of the resulting file? Additionally, if you can, please send the md5sum or checksum, so that one can verify the file integrity. Are you on a Unix (Linux) system? best wishes,

Dear Dr. Compo, I used the following path http://disc.sci.gsfc.nasa.gov/daac-bin/FTPSubset.pl and I got the file named: MERRA300.prod.assim.inst6_3d_ana_Np.20160115.SUB.nc I got from cheksum calculator: 8295317E44DE1615EC88CA9D46838FF7. Probably the file is corrupted because is size is: 1kb. Thank you.

Re: Access to data

Submitted by gilbert.p.compo on Fri, 05/27/2016 - 13:30.

Dear Johanna, Please do let us know how it goes, after trying to the data download again and contacting the email address mentioned above by Mike.

I was just looking into the wind speed profiles provided by MERRA-2. I merged the 3d field with the 10m and 50m wind speed and plottet the resulting profiles. Surprisingly, I found the 50m wind speed being exactly the same value as the lowest level 3d wind speed. The lowest level 3d wind speed is given at an height of about 65m. This is the lowest entry of the mid_level_heights variable.

I am cutting and pasting a response that Mike Bosilovich previously posted. I think these suggestions will be your best bet on getting the most specific answers:

It's very hard to answer your question, since thre is not much information about how you downloaded the data and what exactly the problem may be. First thing to do is to check the file sizes are appropriate. very small files may indicate a corrupted download. Also, this page is not the most efficient for getting a response from the data center or the GMAO. This type of question should go to the GES DISC help page, as it pertains to accessibility of the data.

In particular, the File Specification Document, will tell you everything you need to know about our native data formats and the grid structures (these may be manipulated by GES-DISC utilities though).

Please let us here at reanalyses.org let us know if we can be of more help. Also, if you get an account, you can make a page that illustrates what you are seeing and provides the steps you took to get to your surprising result. (See a page such as http://reanalyses.org/atmosphere/20cr-ensemble-mean-wind-speeds-appear-wrong for a good example of exchanging information about findings).

50m winds are interpolated from the lowest model level using Monin Obukov surface layer similarity theory. So it should be close to, but not exactly equal the lowest model level winds. 1 hourly lowest model level data are included with SLV data collection. (see the LML variables)

Additionally, may I suggest creating a page here at reanalyses.org with figures that illustrates the issue? Then, the page can linked to in the message, and other users could comment on what you are showing. If you don't have an account, direct your browser to http://reanalyses.org/user/register

and then you can create a page for posting a description of your findings and import a few figures.

Gil made some good points, words to live by, as it were. it's hard to answer this question since there are few specifics. Is it temperature, wind or both that are biased? Which days, months season? Which variables are you comparing to METAR? Where is the bias exactly? Warm or cold, fast or slow?

We are working on some regional near surface temperature cold biases over desert surface types, somewhat prominent over the Sahara, but if it is surface type dependent, then it would seem Mexico might be affected similarly. When we have results on that, I will try to remember to open a page on reanalysis.org for it.

This is exactly the sort of detailed information that is needed, except you should specify what is longitude and what is latitude on your grid. I expect it is 60E, 23N to 76E, 39N, but it would be best to confirm. With that addition, I suggest you send your required parameters to gsfc-help-disc@lists.nasa.gov. That is the data center, and they have staff who can address any issues when acquiring MERRA-2 data. In addition, in your message to them describe how you can handle the data. Can you use the netCDF format? Do you need comma delimited data? Are you expecting something else?

You can access the data for your requirements best with the MERRA-2 data subsetter:

https://disc.gsfc.nasa.gov/daac-bin/FTPSubset2.pl

You can select parameter, spatial and temporal subset. You can also select to regrid the data however since the data is 0.625 x 0.5 degree resolution I would only select a grid with a coarser resolution.

To access the data you need to set up an Earthdata account (free) and add "NASA GESDISC DATA ARCHIVE" to the applications. The instructions are in the following links:

I'm trying to extrapolate the roughness lengths from tavg1_2d_flx_Nx (M2T1NXFLX). I'm struggling to understand the datas, from the description the surface roughness should be in [m] but I can't figure how the value are so low. Could you give me some details or text me some references to better understand the values? Many thanks for that.

Just to let you know I'm assuming that a roughness length of 0.002 m is interpretable as a flat surface and a length of 0.8 as an high density metropolitan area.

I can't say for sure if what you are seeing is low or not. The post does not include the values that you are seeing, or their location.

I picked a point in the central US (100W, 40N) and find z0h=z0m=0.0446857m

As far as I know now the documented units of meters for roughness are correct.

If your point is near water, you may have a predominantly water surface which would have small roughness. Check the land fraction variable in the constants file. If you are looking for high density metropolitan areas, keep in mind that the MERRA-2 resolition of ~1/2 degree would include values representative for the whole grid box.

I have just shifted from using MERRA reanalysis dataset to MERRA-2. My observation regards to the U50m, V50m winds from collection tavg1_2d_slv_Nx. The analysis area is Greece. I have noticed a large systematic change in the wind speed calculated for almost adjacent nodes of MERRA2 and MERRA. The "new" winds are about 25% lower than before. The correlation of the wind speeds are ~94% between the data sets but how is this large change explained.

There were numerous model changes from MERRA to MERRA-2, and since 50m is within the lowest model layer, the model physics (especially Similarity profiles) would contribute significantly to the result. (to be sure you understand that a reanalysis is not an observation data set, but uses observations and models to produce the resulting data). Aside from the model changes, this may also depend on the availability and assimilation of winds in the region.

I would suggest starting by reviewing the documentation available, which may provide more context fro the differences between MERRA and MERRA-2. https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/docs/

Next, I would suggest looking at the data provided that can contribute to wind speed. Both MERRA and MERRA-2 provide surface layer stability parameters, transfer coefficients and roughness lengths. These may show some systematic variation that explains the wind speed difference.

Another possibility is that the grid changed from MERRA to MERRA-2. Since you are in a region with both topography and coastlines, the grid points you are looking at may have different geographical characteristics. You should check the constants file for the surface geopotential (altitude) and land/sea fraction. Both of which could dramatically influence surface winds.

Thank you for the prompt reply. Your comments were very helpful. I am aware of the grid point change between the two datasets; my comparison focused on nodes which were really close and the terrain was the same. If I have understood well, you imply that if the comparison is repeated with wind at 850hPa, then the results from the two datasets will converge. So what has changed is the parameterization of surface layer turbulence (scheme of Helfand and Schubert (1995) based on MO similarity vs Louis scheme. This would explain the noticeable deviation of near-surface winds.

I was trying to estimate trends in tropical middle atmosphere. The trends are very small compared to the earlier reports. I look back in the time series then I came to know that there was sudden shift in the winds around 2000. Does it something due to the input data?

Trends typically depend on the variable, and you don't say which variable or which reports. Water vapor, for example, depends greatly on the availability of observations, which changes substantially over the typical satellite reanalysis period. All reanalyses will be subject to uncertainty there. Temperature of the lower troposphere (TLT), however, is fairly well observed, and the reanalyses tend toward the observed values reasonably well. If you are talking about winds, then around 2000 MODIS AMV become available, and could affect time series analysis.

This isn't necessarily a problem for MERRA itself, any of the reanalyses, especially the full satellite observing system reanalyses (including MERRA-2, ERAI, JRA55 and CFSR) should be considered very carefully before assuming a physical forcing behind their trends.

I am looking on middle atmosphere temperature and horizontal wind trends over tropical latitudes. I used 72 level "inst6_3d_ana_Nv" product. Here with i am attaching link, https://sites.google.com/site/kishorekumargrandhi/merra-trends/Annualmean_temperature_8N.png, that shows annual mean temperature over 8 degree North. The annual mean temp. suddenly drop down after 2003. The temperature drop is very drastic. Is it real or any kind artefact in the reanalysis?

Just to be clear, your first post states the question is about MERRA, but this is a MERRA-2 area of the www pages. The figure does not clear that difference, except the path says "merra-trends".

But to answer your question, that is not a trend in the figure, it is clearly a discontinuity, and so not to be trusted. My first suggestion in cases like this is double check your coding to create the figure and data. MERRA does not include monthly or annual data on the model vertical coordinate (Nv), so you likely created these data files on your own. So, it is worth double checking to be sure that the code has not introduced the discontinuity.

There are two possibilities, for a data issue in MERRA to occur at 2003. First, In the early part of the analysis, SSU radiances were assimilated with the GLATOVS radiative transfer model, not the CRTM as AMSU data were assimilated. At the boundary between these, there is a problem, because the SSU were not handeled well by the older GLATOVS code. The other less likely possibility is that AIRS is having some effect there. I only mention it because AIRS will be in full year of 2003.

In either case, MERRA is not the data you want to be using right now. MERRA-2 is available and has a newer CRTM that can handle SSU more seemlessly with AMSU. There isn't an Nv collection for monthly temperature in MERRA-2, either.

To access MERRA data (for example const_2d_asm_Nx, inst6_3d_ana_Np, tavg1_2d_rad_Nx, tavg1_2d_slv_Nx), I developed an Excel Tool in VBA, which allows the download and the conversation to Excel format for selected subsets of this data.
Now I want to enhance this tool to MERRA-2. But this data are password protected and I don't know, how and to which address the login information has to be transmitted from the Excel-VBA procedure to the MERRA-2 Database. Exists a help Information for VBA programmers or generally for Software developers, how to solve the login procedure to access the MERRA-2 database from Software tools, developed for such purposes?
It's no Problem for me, to download the needed data in numerous single steps with the web browser, but this way of conversation by hand into the calculation tools in Excel is much too time consuming and hardly practicable.